casual bicyclists dont know what you mean when you say "on your left". They think it means they are commanded to move left. On the other hand, cyclists know what you mean when you say "on your left" as in you are going to pass them on their leftside. I heard "on your left" over a hundred times in the span of nearly a decade riding. I usually get passed on long descents and relatively flat roads. On climbs its not really necessary to say "on your left" because both parties are moving slow enough that the person about to get passed will notice the rider behind him through peripheral vision. Only idiots ride all over the road and require someone to say "on your left" on a climb. Its definitely necessary to say "on your left" on descents because of the speed. The faster you are moving, the more necessary it becomes. On flat roads you might have to say it if you are moving much faster than the person you are about to pass. If you are barely moving ahead of this person he or she will probably notice you with his or her peripheral vision as you move around the rider.